Hi,

I switched to a raspi 4b and op3.

The question I'm not sure about is: if a switch shortens GPIO 3 (pin 5) to ground while the raspi is running and i2c is active - does this do any harm to the system?

Background: 
1. In OP3 I can configure any free GPIO to be monitored and to trigger a shutdown to halt. Lets assume we choose GPIO 21. GPIO 3 can not be used as it is in use for I2C. Right?
2. The raspi 4b has the wake_on_gpio at GPIO 3(Pin 5) to recover from halt mode. Only GPIO 3 works for this, hard wired, not changeable.
3. If now I have one switch that drives both Pins - GPIO 3 and GPIO 21 - to ground at the same time, what happens?

While the raspi is in halt mode only the ground at GPIO 3 has an effect and makes the raspi wake up. GPIO 21 is not working in halt mode, I see no problem there.

When the raspi is up and running it might get problematic: the GPIO 21 will trigger the shutdown. But at the same time the switch brings GPIO 3 with the active I2C to ground for some milliseconds.
Does that do any harm or will that just be a strange, wrong signal for the I2C?

If it does no harm, this could be mine "one switch for power up and down" solution on board.

Looking foreward for your opions and knowledge....